In the field of medium- and small-size LCDs and OELDs, in particular, portable displays such as mobile devices, digital cameras, and cell phones, an important task is weight reduction and thickness reduction of the displays. In order to realize a further thickness reduction in glass substrates, there has been extensively employed a step in which the surface of a glass substrate is etched after a step of array color filter lamination to reduce the thickness. For example, there has been employed a treatment of etching the surface of a glass substrate having a thickness of from 0.4 mm to 0.7 mm to obtain a glass substrate having a thickness of from 0.1 mm to 0.4 mm.
As etchant for such etching, etchant containing hydrofluoric acid (HF) is generally used because hydrofluoric acid (HF) is superior in etching function against glasses (see patent document 1).
Etchant containing hydrofluoric acid is used also in etching which is conducted for the purpose of making the surface of a glass substrate for an information-recording medium, such as a hard disk drive, into a desired surface having minute projections (see patent document 2).
In order to enhance an etching rate during etching, it would be satisfied by increasing the concentration of hydrofluoric acid in the etchant. However, in the case of increasing the hydrofluoric acid (HF) concentration of etchant, there has been a problem of haze generation on the glass substrate surface.
Patent document 3 discloses an etching method which inhibits haze generation during etching of a thin film of SiOx, SiNx, or the like formed on a glass substrate with hydrofluoric-acid-containing etchant in the production of displays such as LCDs.
Patent document 3 appears to be an invention relating to a method for etching glass, however, the method disclosed therein is not intended for etching a glass substrate itself but intended for etching SiOx, SiNx, or the like deposited on the surface of a glass substrate. This point is evident from the description in patent document 3 (in particular, the description in paragraph [0002]).
When a thin film of SiOx, SiNx, or the like formed on the surface of a glass substrate is etched, the surface of the glass substrate itself is scarcely etched and the etching amount is 1 μm or smaller at the most.
Generally, for the etching of a glass substrate surface, a glass substrate is immersed in etchant. The higher the hydrofluoric acid (HF) concentration of the etchant or the longer the time period for which the glass substrate is immersed in the etchant, the stronger the tendency that haze generates on the glass substrate surface. In other words, the larger the etching amount of the glass substrate surface, the stronger the tendency that haze generates on the glass substrate surface. From this standpoint, the etching for which the invention described in patent document 3 is intended is directed to conditions which are relatively less apt to cause haze generation on the glass substrate surface.
On the other hand, in the case where etching is conducted for the purpose of reducing the thickness of a glass substrate, the glass substrate is etched in an amount of 1-690 μm (usually 10-650 μm). This etching condition is far more apt to cause haze generation on the glass substrate surface as compared with the etching for which the invention described in patent document 3 is intended.
Consequently, in the case where the etching method described in patent document 3 is applied to etching for the purpose of reducing the thickness of a glass substrate, it is unknown whether the method provides the desired haze-inhibiting effect.
As is apparent from the points described above, there has been no etching method intended for reducing the thickness of a glass substrate and which attains a high etching rate and is capable of inhibiting haze generation on the glass substrate surface.    Patent Document 1: JP-A-2003-313049    Patent Document 2: JP-A-2002-237030    Patent Document 3: JP-A-6-340448